This invention is applicable to fields where a laser radar or other device is used to measure atmospheric fine particles, particularly to the industry of atmospheric environmental analysis where atmospheric aerosols and fine particles are observed or detected. The invention is also applicable to meteorological industries and academics where there are needs to monitor and study harmful environmental pollutants such as SPM (suspended particulate matter) and diesel dust that are released and suspended in the atmosphere due, partly to natural phenomena, and partly to various industrial activities and transportation vehicles, as well as scattering volcanic ashes and cedar pollens.
The prior art offers a technology by which two pieces of information about atmospheric fine particles, i.e., their number and size distribution, are determined directly or indirectly using a particle counter (e.g. of a desktop laser scattering type) or an impactor placed on the ground and within a room (see, for example, JIS B9921 1989 “Automatic Particle Counter of Light Scattering Type”; “Handbook of Laser Measurement”, ed. by the Editorial Committee of Laser Measurement Handbook, p. 229–234, 1993; “Latest References for Optical Sensing Technology”, Kazuo Ichijo, Optronics Inc., p. 90–91, 2001; and Japanese Patent Public Disclosure Hei 8-86737). However, it is impossible for those devices to measure directly the number and size distribution of fine particles in the atmosphere either several hundred meters above the ground or several kilometers away from the site of observation. Direct measurement would be possible if the devices were installed in that above-ground or faraway atmosphere but this is not realistic.
The conventional laser radar system is capable of obtaining the information about the fine particles in the faraway atmosphere (see, for example, “Latest References for Optical Sensing Technology—Visualizing the Earth's Atmospheric Environment by Optical Remote Sensing”, Nobuo Sugimoto, Optronics Inc., p. 270–275, 2001; “Handbook of Spectroscopic Technology—Laser Remote Sensing”, ed. by Shigeo Minami and Yoichi Goshi, Asakura Shobo, p. 581–591, 1990; and Japanese Patent Public Disclosure 2001-337029) but even that system cannot directly measure the number and size distribution of atmospheric fine particles since what it does is simply sum the intensities of scattered light from the individual fine particles in the atmosphere and measure a single signal intensity.
The operating principle of the desktop particle counter in the prior art is such that the air continuously drawn into the device is illuminated with laser light or the like and the scattered light beams from the fine particles in the air are counted one by one. By this approach, the number and size distribution of fine particles in a unit volume of air can be measured but not if the air is the atmosphere high above the ground. The information about faraway fine particles can be measured by the laser radar system; however, in the conventional laser radar system, it is impossible to measure the scattered light beams from individual particles in the faraway atmosphere and directly calculate the number and size distribution of the fine particles contained within a limited space. In order to make this calculation possible, the fine particles within a faraway limited space in the atmosphere must be illuminated with laser light and the scattered light beams from the fine particles be detected one by one.